Your Mac desktop background wallpaper image may show up upside down if the photo was taken while holding and iPhone or and iPad upside down.
When the photo is taken, some meta data about the orientation of the camera is saved to the image file causing the image to appear upside down before being exported from iPhoto. Even the photo thumbnail image may appear right-side-up but when the photo is set as a background wallpaper it flips upside down.
The video above shows you how to import the photo into iPhoto and then re-export it to a .jpg file to resolve this issue.
I also attempted rotating the image in preview first but that did not solve the problem.
It is unfortunate that Apple doesn’t include an option to rotate the image in the “System Preferences”.
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